In modern turbochargers, single-stage centrifugal compressors with vaned diffusers are used to increase the induction pressure of the engine. In the diffuser, the kinetic energy of the medium which is to be compressed is converted into static pressure. The compressor impellers comprise a specific number of impeller blades, and the diffusers have guide vanes with prismatic, aerodynamic profiles (i.e., wedge-shaped or airfoil-shaped). As seen in the direction of the compressor axis, the guide vanes have a determined tangential angle at the leading edge (inlet angle), a determined tangential angle at the trailing edge (exit angle), and a determined spacing in the circumferential direction between two guide vanes which are arranged adjacently to each other.
When designing compressor stages, a compromise must constantly be found between the aerodynamic performance, the mechanical load and the development of noise by the compressor. Modern compressor stages with high specific swallowing capacities have long, thin impeller blades, the natural modes of which occur at low frequencies and can easily be excited and set in oscillation. A primary source of these excitations is a pressure potential field which is created by the guide vanes of the diffuser. On account of the geometrically regularly formed compressor impeller blades and diffuser guide vanes, resonance vibrations can occur which become effective as vibrational energy increases. On account of the high speed which is required for achieving the discharge pressure, the vibrations can lead in the extreme case to mechanical damage (high-cycle fatigue—HCF) in the compressor impeller blades.
EP 1 772 596 discloses a two-stage diffuser of a centrifugal compressor, in which a diffuser vane row is arranged in front of and behind the impeller blades of the compressor which are exposed to axial throughflow. The diffuser vane rows have a different number of guide vanes each in a lower and an upper half in order to reduce the vibration influence, as a result of the uneven vane concentration, upon the impeller blades which are arranged between the diffuser vane rows. The throughflow takes place in the axial direction, wherein the flow conditions in front of and behind the represented two-stage diffuser along the circumference are symmetrical. Such symmetrical flow conditions are necessary since the axial turbine, which is to be fed a flow which is as symmetrical as possible along the circumference, is arranged downstream of the diffuser.
U.S. Pat. No. 3,873,231 discloses a liquid pump with an impeller with a multiplicity of blades and a plurality of guide vanes which are arranged in a distributed manner along the circumference. The guide vanes in this case have an uneven distribution along the circumference in order to be able to avoid constant pressure patterns in the liquid which is to be pumped.